The present invention relates generally to liquid filtration by means of a filter cloth. In particular the present invention relates to the filtration of waste water which discharges from the final purification stage of a waste water purification plant.
Filtering devices for liquids operating with filter cloths are known in different designs, e.g. in the form of filter presses, disk filters (EP-B-0 413 178) or drum filters. The invention is applicable in the case of filtering devices of any design. The filter cloths used in such devices are single-layer fabrics which behave like a sieve, the holes of which are defined by the mesh of the fabric. However, as a rule filter cloths have a multi-layer structure comprising a supporting fabric with a relatively large mesh and a filter-active layer. In the filler-active layer the threads or filaments lie as multiple plies over one another. A deep or spatial filter of this type has angled and multiply curved flow channels within which solid particles, which are smaller than the free cross-section of the spaces between the threads, can be held back, partially by mechanical blocking in the flow channels and partially by adsorption on the walls thereof. Such a filter-active layer with a deep filter effect as a rule is made as a needle felt or tangled fiber fleece.
The flow resistance of such a filter cloth is relatively high and increases quickly with increasing blockage of the flow channels in the filter layer. Therefore the filter cloth must be cleaned at relatively short intervals. As a rule this is performed by back-rinsing, that is, by passing liquid, such as, e.g. fresh water or filtrate, back through the filter cloth in the direction opposite to the direction of filtering. If the side of the filter cloth against which the liquid flows in the filtering operation is designated as the front side and the other side as the back side, then the back washing flow necessary for back washing the filter cloth can be created by sucking liquid on the front side or supplying liquid under pressure on the back side of the filter cloth, or both. The back washing can be carried out on the entire surface of the filter cloth simultaneously or only on a partial area of the filter cloth, which is moved back and forth over the entire surface thereof. An example of a suction device movable over the filter cloth for back washing is described in the already mentioned EP-B-0 413 178.
Those solid particles, which are deposited on the outside of the filter cloth at the time of filtering and form a layer of the pre-coat layer type which supports the filter action, can easily be removed by back washing. However, those solid particles which are caught and blocked in the numerous narrow and angled flow channels of the filter-active layer of the filter cloth, produce considerable problems at the time of back washing. These problems are greater the more the layer has the nature of a deep filter. In the twisting flow channels of the filter-active layer no flow speed can be applied which is so great that the solid particles are completely released and washed away. Limits are set on the increase in the pressure or, as the case may be, on the flow speed since the filter cloth is damaged at too high a speed. The filter cloths known up to now where the effective filter-active layer is made as a deep filter, e.g. as needle felt or tangled thread fleece, cannot be cleaned without residue remaining therein. These become clogged so that they have to be replaced after a relatively short operating time.
More advantageous is the formation of the filter cloth as a so-called pile material, that is, a pad, plush, or velvet fabric, which consists of a relatively large-meshed supporting fabric and, for example, a pile made of pile threads worked into the W binding. In the filter operation the pile threads lie on the upstream side of the filter cloth and their length, stiffness, and thickness are selected so that the pile threads are shifted by the flowing liquid into a position approximately parallel to the supporting fabric. The pile threads lie in multiple layers or plies over one another and form a deep filter containing numerous narrow, angled, and multiply curved flow channels between the pile threads lying over one another. Solid particles, smaller than the unobstructed width of the flow channels, can be held therein partially mechanically and partially by adsorption. On the other hand, at the time of the back washing the pile threads are exposed to a liquid flow directed away from the supporting fabric and in this way are raised relative to the supporting fabric. Then they are largely parallel to one another and to the direction of flow, by which the volume of the pile layer is greatly enlarged and the flow channels existing between the pile threads are opened and leveled. By means of this, on the one hand, the flow resistance for the back washing liquid is reduced so that it can flow between the pile threads at a high rate, and on the other hand the solid particles mechanically blocked previously in the flow channels are released, so that they can be loosened and washed away by the back washing liquid, overcoming the adsorption forces.
Processes and devices for filtering liquids, with the use of a pile material as a filter medium and with the use of the previously described straightening effect at the time of back washing are known from FR-A-2 366 864 and NL-A-81 03 750. In the case of the process and the device in accordance with NL-A 81 03 750 it is known that the back washing which causes the straightening of the pile threads is created by means of a suction bar with a suction slit, which stands opposite the pile side of the filter cloth at a short distance and is moved parallel to this.